<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        let year = Number(prompt('请输入年份'))
        let month = Number(prompt('请输入月份'))
        if (month === 1 || month === 3 || month === 5 || month === 7 || month === 8 || month === 10 || month === 12) {
            alert('这个月份31天')
        }
        else if (month === 2) {
            if (year % 400 === 0 || year % 4 === 0 && year % 100 !== 0) {
                alert('这个月份29天')
            } else {
                alert('这个月份28天')
            }
        } else if (month === 4 || month === 6 || month === 9 || month === 11) {
            alert('这个月份30天')
        } else {
            alert('没有这个月份')
        }
    </script>
</body>

</html>